It is known that tractors are often used to tow aircraft on the ground from one location to another. Such tractors typically use a tow bar with a coupler on one end and a tow bar head on the other end. The coupler couples to the hitch of the tractor and the tow bar head couples to a nose gear of the aircraft to be towed. Aircraft nose gears have a certain threshold force value that cannot be exceeded during the push and pull phase of a tractor maneuvering the towed aircraft on the ground. Exceeding this threshold force value can damage the nose gear/landing to the extent that it can malfunction after take-off, or render it unable to cycle correctly making it very dangerous for operation. If such damage to the nose gear is detected, the nose gear is required to be tested and certified prior to flight departure. This repair of the nose gear and testing for certification prior to any use in flight represents a significant monetary expense, including the time expended that the aircraft is unable for commercial use. If the damage to the nose gear goes undetected, it can result in serious problems during flight/take-off/landing that could, unfortunately, result in a crash or other dangerous situations. Accordingly, it is very important for aircraft tractors to tow aircrafts without exerting forces that would damage or wear away at the nose gear or nose gear shear pins. Unfortunately, prior art methods and devices are lacking.
Use of aircraft tractors to move aircrafts, rather than the aircraft's own engines, became desirable in order to save on fuel. Initially, tractor speeds during towing efforts had been previously restricted to very slow speeds (i.e., less than 10 miles/hour). It became desirable to increase such speeds, even for fully loaded aircrafts, to increase the viability of taxiing fully loaded aircrafts by tractor, rather than the aircraft's engine, for additional fuel saving. However, this would increase forces between the aircraft and the tractor, which could result in too much force being transmitted to the nose gear. In addition, such higher speeds would increase the risks associated with jack-knifing. Also, it was not desirable to leave the control over the aircraft solely in the hands of the tractor driver, who may not have been as skilled and responsible as a pilot.
One solution was developed in U.S. Pat. No. 4,113,041 (Birkeholm), which included a sensing element within an aircraft tow bar that measures the traction and pressure forces transmitted between the tractor and the aircraft and a control unit that automatically adjusted the traction-effect of the tractor, as well as, the braking of the aircraft, to keep the tow bar forces within a predetermined permissible level. For safety, the Birkeholm system included an emergency release system that separated the aircraft from the tractor in response to the tow bar forces exceeding a predetermined range. Unfortunately, there are some major drawbacks to this solution. By being integrated with the aircraft and the tow bar, the Birkholm system was not flexible for use with other aircraft, as well as, other tow bars. In addition, some drivers and pilots are not comfortable with such automatic traction adjustment and aircraft braking controlled by an electronic control unit, which is subject to error.
In order to avoid costly damage to the nose gear by transmitting forces that are too high for the nose gear to sustain, it is also known to use a towing pin, also referred to as a shear pin, as a mechanical fuse so that such high forces can break the shear pin, rather than the nose gear. However, this solution is not perfect. In order to guarantee that a shear pin will break when the situation requires it, such as when the steering angle of the wheels on the nose gear is considerable, sheer pins are manufactured with a rather large safety margin, which leads to pins breaking frequently, and sometimes unnecessarily. In addition, even though breaking the shear pin breaks the connection between the aircraft and the tow bar (and, thereby, the tractor), which terminates the application of excessive forces to the nose gear, it is nonetheless a hazard as the aircraft may not completely come to a rest immediately after the connection is terminated, and may continue to roll on its own momentum.
One solution was developed in U.S. Pat. No. 9,108,746 (Schmidt), which disclosed a shear pin with a force sensor and an accelerometer that measured the shear pin's angle of inclination. Schmidt's shear pin also calculated a force threshold as a function of the measured force and the angular orientation of the nose gear wheels, as determined by the accelerometer, generating an alarm when the force threshold was reached or exceeded.
Unfortunately, these prior art devices and systems are specific to a particular aircraft. Such devices and systems are not adaptable and flexible so that a single instance of a device/system may be tied to the tractor and configured for interchangeable use, on-the-fly, with multiple aircrafts, multiple shear pins, and multiple tow bars.
In particular, towhead shear pins are fitted to an individual aircraft towhead and are manufactured so as to shear upon a predetermined force being met or exceeded. Such shearing of the shear pin is intended to be an alert to the tractor driver that a safe force has been exceeded and inspection, as well as, replacement of the shear pin is required. Accordingly, failure of the shear pin itself is undesirable.
Therefore, a need exists to overcome the problems with the prior art as discussed above.